Gas and steam turbine systems are predominantly used to generate energy. Here a modern gas and steam turbine system usually includes one to four gas turbines and at least one steam turbine, with either each turbine powering a generator in each instance (multishaft system) or a gas turbine with the steam turbine on a shared shaft powering an individual generator (solid turbine system). The hot exhaust gases of the gas turbine are used here in a heat-recovery steam generator to generate steam. The steam is then fed in to the steam turbine. Approximately two thirds of the electrical power is usually allotted to the gas turbine and a third to the steam process.
The power of conventional gas and steam turbine systems lies for instance in the range between 80 MW and 830 MW per unit of gas turbine/steam turbine, with it being possible for a power plant to consist of several units. In comparison, a block of a nuclear power plant usually has a power between 500 MW and 1500 MW.
To increase the efficiency of the steam turbine of the gas and steam turbine system, this usually includes several pressure stages, which are configured for different steam pressures. In this way, the steam which is superheated in the heat-recovery steam generator with the aid of the exhaust gas of the gas turbine is fed back into the heat-recovery steam generator after its decrease in tension in the high pressure stage of the steam turbine and is superheated again. This steam is then fed to the downstream pressure stages. This so-called intermediate superheating increases the efficiency of the steam turbine above the higher average temperature of the heat supply. Incidentally, erosion on the last blades in the low pressure part of the steam turbine is therefore prevented as a result of excessively high wetness of steam.
A plurality of pressure stages in a steam turbine is theoretically conceivable, however the additional capital expenditure is then too high in proportion to the thermotechnical improvement. A gas and steam turbine system with three pressure stages in the steam turbine and a single intermediate superheating currently represents the economical optimum.
As a result of its flexibility, in particular the possibility of a rapid start-up, modem gas and steam turbines are often used to provide briefly occurring high power needs in the mains supply, so-called peak loads. This results in comparatively frequent start-up processes in gas and steam turbine systems. To start up the gas turbine, the generator assigned to the gas turbine is usually used here as an engine, with the speed of the gas turbine slowly being increased with the aid of a system-specific characteristic curve. To this end, a starting device with a frequency converter can usually be connected upstream of the generator, said frequency converter being able to generate power at any frequency from the mains network.
In the state of permanent operation, the turbines of the gas and steam turbine system usually rotate with the mains frequency of 50 Hz or 60 Hz. Operating states may however also be desired or needed, in which the steam turbine rotates in particular with a minimal speed, during a bridging of the high pressure stage of the steam turbine on the steam side or during the heating process.
In the case of lower speeds as the power frequency, the generator of the steam turbine must however be decoupled here from the mains supply. Accordingly, no power is taken by way of the generator in these operating states, as a result of which the counter torque acting on the steam turbine is comparatively minimal and the steam turbine can in some circumstances not be kept at a constant speed. This nevertheless indicates a loss of flexibility in respect of the mode of operation of the overall gas and steam turbine system, and possibly unfavorable operating points (for instance retaining speeds, which are close to the natural frequencies) are taken into account, which could result in unnecessary damage and ageing of the steam turbine.